1. Field of Invention
This invention relates to a method of measuring and controlling radiant heater surface temperatures that would otherwise be very difficult or expensive to measure or control. It is particularly useful for heaters and furnaces such as UV or microwave dryers which use plasma bulbs as heat sources.
2. Description of Prior Art
UV furnaces are used for drying and curing adhesives, inks and other coatings. Most of these furnaces have radiant heat sources such as RF and microwave excited plasma bulbs. Efficient operation of the plasma bulbs require that material inside the bulb be in a gaseous and ionized state. Many of the desired materials condense at less than 357° C. so these bulbs have starter material necessary for ignition. After ignition and heating, the desirable materials vaporize and become active. Plasma bulbs emit UV radiation which is part of the electromagnetic spectrum and is limited by X radiation on the short side and infrared light on the longer wave area. The wave length of the light is determined by the chemistry of the excited gases. Heat from these radiant sources is across a wide spectrum: UV (100 to 380 nm), visible light and IR. The enclosure that surrounds the bulb can have selective coatings that determine the wave length of the reflected light, (US Patent 2005/0115498 A1)
The plasma bulbs will overheat and fail without proper cooling. Cooling air is forced by a fan or blower over the heat source. Radiant heat from the source is usually reflected and/or focused onto the material to be dried or cured. Sometimes shields are added to prevent convective heating of the material to be cured. (U.S. Pat. No. 6,831,419)
The body of the plasma bulb is usually made of quartz. Quartz has a very low coefficient of thermal expansion and thus is almost immune to thermal stress. It is nearly transparent to the IR and UV radiation so radiant heating is small. However the quartz tube is the weak link and it has been the focus of much innovation. U.S. Pat. No. 5,541,475 showed that varying the quartz wall thickness can compensate for uneven heating.
Practical use of these dryers can require intermittent operation. Such operations require a shutter system (as taught in U.S. Pat. No. 6,933,683). The shutter system seals the optical cavity and reduces heat loss when the bulb is operated at reduced power. Alternately, the bulb power can be reduced and abruptly restarted (U.S. Pat. No. 5,838,114). The time response of the cooling systems and quartz bulbs are not equal and uneven heating and cooling degrades the bulb.
Another technique in dryer operation is to temporarily overdrive the heater. Then the power is reduced before the bulb overheats. The intermittent high power allows a more efficient operation. (U.S. Pat. No. 6,690,112)
Bretmersky et al (US 2008/0017637) describe control systems for UV dryers. The inventors recognize that reliable operation requires that the heater element be actively cooled. The technique developed by them was to estimate cooling capacity of the air steam. It is well understood that convection heat transfer can be correlated to coolant velocity which can be correlated to system pressure drop. Then by using the estimated velocity, fluid properties, surface conditions and local conditions (velocity, boundary layer thickness, etc.), the heat transfer coefficient can be inferred. That calculated heat transfer coefficient and estimated temperature difference is then used to calculate the convection cooling capacity. Next the cooling requirements of the source are estimated based on that bulb's power density. And finally with this information, the cooling (air speed) is adjusted by the control system to provide the required cooling. Such a system is at best an estimate on indirect measurements.
An alternate embodiment as described by Bretmersky utilizes pressure and temperature measurements. A temperature sensor measures the coolant temperature but with some radiant heating from the heat source. Radiant heating of the sensor is a function of the fourth power of the radiant heat source temperature. It is inversely proportional to the square of the distance from the heater. Radiant heating of the temperature sensor is also affected by its absorptivity. This makes the temperature of questionable value in estimating the radiant heat source temperature. It is possible to design a system where the sensor reports the heater temperature correctly but that will be at only one condition. Changing the inlet air temperature, distance from heat source, absorptivity or heat source power will produce erroneous readings. The primary purpose of this system is to control the radiant heat source temperature but that temperature is not measured.
Surrogate temperature sensors have been used in other industries as taught by Potts (U.S. Pat. No. 6,726,401) in measuring the temperature of a leach field. He used the temperature of an adjacent field that was convenient to measure.
In the case of temperature measurements in a harsh environment, such as a diesel engine combustion chamber, Abe et al (U.S. Pat. No. 4,516,543), showed how a circuit could simulate the temperature of the object to be controlled. The simulated temperature was used as a surrogate temperature when controlling the diesel engine glow plug. This patent showed how to estimate and then control glow plug temperature with a simulated glow plug temperature.